130 Prof. T. Carnelley on the Colour 



of hemihedrism, if observed, would be accompanied by circular 

 polarization *. The crystals of sal-ammoniac did not show it. 

 In the case of cuprite, although many crystals transmit 

 light in irregular patches between crossed Nicols, there is no 

 evidence of circular polarization. 



Note. — The cleavage of cuprite is stated in the text-books to 

 be octahedral. The specimens examined on the present occasion 

 show, in addition to the uneven and interrupted octahedral 

 cleavage, a bright and perfect cleavage parallel to the faces 

 of the cube. 



XVI. On the Colour of Chemical Compounds, chiefly as a 

 Function of the Atomic Weights of their Constituent Elements. 

 — Parti. Inorganic Compounds. By Thomas Carnelley, 

 D.Sc. (London), Professor of Chemistry in University 

 College, Dundee^. 



THERE are at least three circumstances which condition 

 the colour of chemical compounds, viz.: — - 



1. Temperature. 



2. The quantity of the electro-negative element present in 

 a binary compound. 



3. The atomic weights of the constituent elements of the 

 compound. 



Of these, the first two, together with a few other points in 

 this connexion, have been studied in some detail by DelavalJ, 

 Talbot §, Brewster ||, SchonbeinU", Gladstone**, Houston tt? 

 AckroydJt, Petrie§§, Ross||||, and Bayleylffj". Though it is 

 to the third of these circumstances that I wish more particu- 

 larly to direct attention in the present communication, yet it 



* Groth, Physikalische Krystallographie, 1876, p. 223 ; Von Lang, 

 Lehrbuch der Krystallographie, 1866, p. 114. 



t Communicated by the Author. 



\ li An experimental Enquiry into the Cause of the Permanent Colours 

 of Bodies," Manchester Phil. Soc. Mem. ii. pp. 147-272 (1789). 



§ « Chemical Changes in Colour," Phil. Mag. [3] ii. p. 359 (1833). 



|| " Colour of Natural Bodies," Phil. Mag. [3] viii. p. 468 (1836). 



5[ "Cause of the Change in Colour which many Substances exhibit 

 under the Action of Heat," Pogg. Ann. xlv. p. 263 (1838) ; " Colour 

 Changes," Erdm. Prah. Chem. lxi. p. 193 (1854) ; " On the Influence of 

 Temperature on the Colour of Substances," Basel Verhandl. i. p. 13 (1857). 



** " Effect of Heat on the Colour of Salts in Solution," Phil. Mag. [4] 

 xiv. p. 423 (1857) ; il Colour of Chloride of Copper in different States of 

 Hydration," Chem. Soc. Journ. viii. p. 211 (1856). 



ft <l Change of Colour produced in certain Chemical Compounds by 

 Heat," Franklin Instit. Journ. lxii. pp. 115-127 (1871). 



XX u Metachromatism," Chem. News, xxxiv. p. 76 (1876) ; Nature, xiii. 

 pp. 298, 385 (1876) ; "Selective Absorption," Phil. Mag. [5] ii. p. 423 (1876). 



§§ "Metachromatism," Nature, xiii. pp. 347, 426 (1876). 



|| || Chem. News, xxxiv. pp. 108, 132 ; also ' Pyrology,' p. 114. 

 % 11 " Colour Properties and Colour Kelations of the Iron-Copper Group," 

 Chem. Soc. Journ. xxxix. p. 362 (1881). 



